1. Field of the Invention
The present invention relates to an operating screw for causing a counterpart member to reciprocate in a prescribed direction. The present invention also relates to a driving mechanism incorporating such an operating screw.
2. Description of the Related Art
As known in the art, an electrophotographic printer includes a photosensitive drum upon which the latent image of desired graphic matter is produced. The latent image is developed by a resinous powder (toner), and then the visible image is transferred onto recording paper.
FIG. 5 of the accompanying drawings shows some of the principal components of a conventional electrophotographic printer. Specifically, the printer includes a photosensitive drum D and corona wires W extending in parallel to the rotation axis of the drum D. In operation, the cylindrical surface of the drum D is electrically charged by the corona wires W. The charged surface is then exposed to light, to produce a latent image of the desired graphic matter. The latent image is developed by toner, and the visible image is transferred onto recording paper.
As shown in FIG. 5, the conventional printer is provided with a reciprocative cleaning unit Uxe2x80x2 for the corona wires W. The cleaning unit Uxe2x80x2 includes cloth pads M attached to supporting members 82 in a manner such that a pair of cloth pads M sandwiches one of the corona wires M. The supporting members 82 are fixed to the frame 81 of the unit Uxe2x80x2.
The cleaning unit Uxe2x80x2 is caused to reciprocate longitudinally of the drum D by a driving mechanism 100a (to be described below). Upon reciprocation of the cleaning unit Uxe2x80x2, each pair of the cloth pads M is moved in sliding contact with the relevant one of the corona wires W. In this manner, it is possible to wipe off dust (including toner powder) accumulated on the wires W.
Referring to FIGS. 6A-6B, the driving mechanism 100a includes an operating screw 100 and a hollow cylindrical carriage 5. The screw 100 extends in parallel to the corona wires W (see also FIG. 5) and is fitted into the carriage 5. The carriage 5 is fixed to the frame 81 of the unit Uxe2x80x2. As illustrated, the screw 100 is formed with a spiral groove 111, while the carriage 5 is formed with a spiral ridge 51 coming into engagement with the groove 111 of the screw 100. Upon rotation of the screw 100, the carriage 5 is moved along the screw 100, whereby the cleaning unit Uxe2x80x2 is moved axially of the drum D. Thus, the cloth pads M (FIG. 5) wipe off the unwanted dust on the corona wires W.
Usually, an operating screw of the above type is produced by subjecting a solid metal cylinder to mechanical processing for forming a spiral groove in it. However, the thus obtained screw often proves expensive due to the material cost and manufacturing cost.
To reduce the overall costs, the conventional operating screw 100 is fabricated by a method which does not employ a mechanical processing for making the spiral groove. Specifically, the operating screw 100 consists of a solid cylindrical core and a resin outer member (formed with the spiral groove 111 mentioned above). The grooved outer member is produced by injection molding using a die 109 consisting of four parts 191-194, as shown in FIG. 7. In the assembled state around the cylindrical core, the four parts 191-194 in combination define a cavity into which molding material (molten resin) is supplied. After the material solidifies, the molding parts 191-194 are removed. Thus, the outer member formed with the desired groove 111 is produced around the core. In accordance with such injection molding, the operating screw 100 is obtained more easily and more inexpensively than by the mechanical processing.
While the conventional screw 100 has the above-noted advantages, it still suffers from the following drawbacks.
As noted above, the outer member with the spiral groove 111 is produced by using four molding parts 191-194. In this manner, however, the resulting screw 100 tends to be formed with some casting fins or burrs B at places corresponding to the parting lines L, as shown in FIGS. 6B and 7. As readily understood, the burrs B result from the supplied molten resin flowing into the gaps formed between the neighboring mold parts 191-194 (see FIG. 7). Some of the burrs B may be formed in the spiral groove 111 (as in FIG. 6B), while others may be formed on the threads of the screw 100 (as in FIG. 7).
Disadvantageously, the burrs B may interfere with the inner surface or spiral ridge 51 of the carriage 5, thereby hindering the rotation of the screw 100. To avoid this drawback, the burrs B may be removed at a prescribed stage in the fabrication procedure with the use of e.g. a grinding device. However, such an additional grinding operation is time-consuming and lowers the production efficiency.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an operating screw which is inexpensive and can be readily made. Another object of the present invention is to provide a driving mechanism incorporating such an operating screw.
According to a first aspect of the present invention, there is provided an operating screw includes: a core having a rotation axis; and an outer member enclosing the core and formed with a spiral groove. The outer member includes a first slide surface whose center of curvature resides on the rotation axis of the core. The first slide surface has a predetermined radius of curvature. The outer member also includes a first retreat surface. The distance between the retreat surface and the rotation axis is smaller than the radius of curvature of the first slide surface.
Preferably, the outer member may be made of a resin material by injection molding.
Preferably, the first retreat surface may be flat.
Preferably, the outer member may include a second slide surface whose center of curvature resides on the rotation axis of the core. The second slide surface may have a radius of curvature which is equal to the radius of curvature of the first slide surface.
Preferably, the first slide surface and the second slide surface may be spaced from each other about the rotation axis of the core, with first retreat surface intervening between the first and the second slide surfaces.
Preferably, the outer member may include a flat second retreat surface separated from the first retreat surface by the spiral groove.
Preferably, the first and the second retreat surfaces may be aligned with each other based on a reference line parallel to the rotation axis of the core.
Preferably, the spiral groove has a maximum width at a position corresponding to the reference line.
Preferably, the spiral groove may be provided with a cutout at a position corresponding to the reference line to realize the maximum width.
According to a second aspect of the present invention, there is provided a driving mechanism which includes: an operating screw provided with a spiral groove and with a spiral projection defined by the spiral groove; and a hollow cylindrical carriage provided with threads coming into engagement with the spiral groove of the screw. The spiral projection is provided with both a plurality of curved surfaces spaced from each other and a plurality of flat surfaces alternating with the curved surfaces.
Preferably, the driving mechanism of the present invention may further include both a guide rod parallel to the operating screw and a slider slidable on the guide rod. The carriage is linked to the slider.
According to a third aspect of the present invention, there is provided a printer which includes; a photosensitive drum; a charging corona wire arranged along the drum; a cleaning member held in contact with the corona wire; and a driving mechanism that moves the cleaning member longitudinally of the corona wire. The driving mechanism includes an operating screw provided with a spiral projection. The spiral projection includes both a plurality of curved surfaces spaced from each other and a plurality of flat surfaces alternating with the curved surfaces.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.